Comparative evaluation of critical operating conditions for a tubular catalytic reactor using thermal sensitivity and loss-of-stability criteria

Optimal operation of a chemical reactor according to various performance criteria often drives the system towards critical boundaries. Thus, precise evaluation of runaway limits in the parametric space becomes a crucial problem not only for the reactor’s safe operation, but also for over-designing the system. However, obtaining an accurate estimate for operating limits is a difficult task due to the limited validity of kinetic models describing complex processes, as well as the inherent fluctuations of the system’s properties (catalyst, raw-material quality). This paper presents a comparison of several effective methods of deriving critical conditions for the case of a tubular fixed-bed catalytic reactor used for aniline production in the vapour phase. Even though the methods being compared are related to one another, the generalised sensitivity criterion of Morbidelli-Varma (MV) seems to be more robust, not depending on a particular parameter being perturbed, when compared to the criteria that detect an incipient loss of system stability in the critical region (i.e., div-methods based on the system’s Jacobian and Green’s function matrix analysis). Combined application of div- and MV criteria allows for an accurate evaluation of the distance from the reactor’s nominal conditions to the safety limits.     

Study of the separation of lactose,lactulose and galactose by liquid chromatography using cationic ion-exchange resin columns

Summary A strong acid cation-exchange resin, Dowex AG50W-X8 in potassium, calcium or sodium form, with water as eluent, has been used to separate three sugars (lactose, lactulose and galactose). The linearity of lactulose equilibrium isotherms was verified. Thermodynamic and kinetic parameters were thus determined by the moment method. Evaluation of the parameters revealed in this work can be considered as the first step in scaling up the process towards industrial use. On the other hand, the equilibrium stage model, which takes mass transfer resistance into account, is shown to provide a fair representation of the system’s behaviour.     

Electrochemical impedance spectroscopy studies of the Ag(111)/<Emphasis Type="Italic">n</Emphasis>-decanethiol monolayer/NaNO<Subscript>3</Subscript> interface

Surface properties of a modified single-crystal silver (111) face are studied in aqueous 1 M NaNO₃ solutions with different acidity in the presence of a monolayer n-decanethiol (DT) film by the methods of electrochemical impedance spectroscopy (EIS) and cyclic voltammetry that employs the meniscus contact technique. It is shown experimentally that in the potential range from 0 to −0.5 V, a DT-modified silver electrode in NaNO₃ solutions with pH 6 behaves as an ideal capacitance in a wide frequency range (from 0.01 Hz to 100 kHz). It is found that with the increase in the solution acidity up to pH 4 and 2, the region of stable capacitance characteristics narrows down. The concomitant changes in the adsorbed monolayer structure lead to deviations of impedance characteristics measured using a low-frequency ac signal from those of the ideal capacitance. A potential shift to values more negative than −0.5 V leads to the destruction of the monolayer. Based on the analysis of experimental EIS characteristics, system’s deviation from the ideal behavior that may indicate violation of the layer continuity is assessed and the relaxation frequencies of the DT/Ag(111) interface in solutions of different acidity are calculated. Estimates of capacitance, resistance, and thickness of the adsorbed monolayer are obtained as a function of the solution pH.     

Two-frequency criterion of the presence of inductive component in the electrochemical cell impedance

This work is dedicated to improvement of accuracy of formal resistor- and capacitor-containing equivalent circuits by introducing inductivities therein. Three signs that evidence presence of inductive component in the electrochemical cell impedance are reported. A simple algebraic criterion is suggested in the form of inequality; it allows detecting hidden inductivity by the results of the measuring of sample’s capacitance and conductivity at two frequencies. Physical meaning of the criterion is analyzed; it was tested by example of three equivalent circuits of which two included the inductivity. The discussed signs were used in the analyzing of data of the impedance spectroscopy of ferro-pseudobrookite and corundum ceramics. It is shown that with the increasing of temperature the inductive features became more pronounced for all samples, which allows their registering with the suggested procedure of the experimental data processing.     

Relaxation spectra of the Warburg impedance and the constant-phase elements

It is shown that there is indissoluble connection of impedances of constant-phase elements (CPE) with the presence, in a system, of a continuous spectrum of intrinsic frequencies inside a region where an impedance of CPE is observed. Such a connection exists in objects of any nature. As a result, the inevitable presence of the minimum limiting relaxation frequencies in real systems distorts the frequency characteristics of an ideal impedance of CPE in the zone where intrinsic frequencies are absent (restricts CPE). A transition zone of frequency characteristics of restricted CPE is studied. A false effect of the deviation of the CPE power index determined experimentally from a Nyquist plot from its true value is shown. A technique is proposed for restoring true values of CPE power indices and estimating limiting frequencies of relaxation spectra lying by many orders of magnitude lower than the minimum frequencies available for an experimental setup.     

Thermodynamic,Dielectric and Conformational Studies on Hydrogen Bonded Binary Mixtures of Propan-1-ol with Methyl Benzoate and Ethyl Benzoate

Dielectric relaxation studies of propan-1-ol with alkyl benzoates (methyl benzoate and ethyl benzoate) have been carried out, for various mole fractions, at different temperatures using a LF impedance analyzer, Plunger method, and an Abbe’s refractometer in the radio, microwave and optical frequency regions, respectively. Kirkwood’s effective correlation factor, the corrective Kirkwood correlation factor, Bruggeman parameter, relaxation time, excess inverse relaxation time and thermodynamic parameters were calculated using the experimental data. Conformational analysis of the formation of hydrogen bonds in the equi-molar binary mixtures of propan-1-ol with alkyl benzoates is supported by experimental and theoretical FT-IR values.     

Computational study of pulsed neutron induced activation analysis of cargo

Pulsed neutron induced activation analysis is a nondestructive technique to detect threats hidden in bulk objects such as cargo pallets, trucks, etc. Isotopic content of cargo can be measured by counting photons emitted with characteristic energies as a result of neutron induced reactions within cargo’s materials. Neutron and gamma radiation transport in active interrogation system consisting of a 14-MeV neutron source, photon detector, and a cargo truck was analyzed with MCNPX code. Gamma ray signatures of cargo with hidden explosive threat were analyzed during the neutron pulse and between neutron pulses for varying system’s geometry and material composition of cargo.     

Thermal decomposition kinetics of PrMO<Subscript>3</Subscript> (M = Ni or Co) ceramic materials via thermogravimetry

Thermogravimetric data using the non-isothermal kinetic models of Flynn and Wall and “Model-free Kinetics” were used to determine the activation energy to study the decomposition kinetics of the ligand groups with system’s metallic ions that takes part in the synthesis of PrMO₃ (M = Ni or Co). This activation energy was determined for the stage of highest decomposition of the organic matter to establish parameters in synthesis condition optimization and application of the proposed material.     

Dielectric and Thermodynamic Studies on the Hydrogen Bonded Binary System of Isopropyl Alcohol and Aniline

The molecular interactions between the polar systems isopropyl alcohol (IPA) and aniline for various mole fractions at different temperatures were studied by determining the dielectric permittivity using an HP-LF impedance analyzer at radio frequencies, the plunger method in the microwave frequency range and Abbe’s refractometer in optical region. Kirkwood effective correlation factors, corrective Kirkwood correlation factors, excess permittivities, Bruggeman parameters, excess Helmholtz free energy, relaxation time, dipole moment and excessive dipole moment were calculated using the experimental data. Optimized geometries were calculated using Spartan Modeling software for both pure and equimolar systems of isopropyl alcohol and aniline for Hamiltonian quantum mechanical calculations. Conformational analysis of the formation of hydrogen bond between the two systems is supported by the FT-IR spectra.     

Vibrational energy relaxation of small molecules and ions in liquids

A theoretical/computational framework for determining vibrational energy relaxation rates, pathways, and mechanisms, for small molecules and ions in liquids, is presented. The framework is based on the system—bath coupling approach, Fermi’s golden rule, classical time-correlation functions, and quantum correction factors. We provide results for three specific problems: relaxation of the oxygen stretch in neat liquid oxygen at 77 K, relaxation of the water bend in chloroform at room temperature, and relaxation of the azide ion anti-symmetric stretch in water at room temperature. In each case, our calculated lifetimes are in reasonable agreement with experiment. In the latter two cases, theory for the observed solvent isotope effects illuminates the relaxation pathways and mechanisms. Our results suggest several propensity rules for both pathways and mechanisms.     

Ultrasonic studies of polysaccharide (pectin) in aqueous media at 298.15 K

The ultrasonic velocity, density and viscosity of pectin in aqueous medium were measured at 298.15 K. The acoustical parameters such as adiabatic compressibility (β), free length (L _f), free volume (V _f), internal pressure (π_i), acoustical impedance (Z), and relative association (R _A), Rao’s constant (R) and Wada’s constant (W) are calculated. The results are interpreted in terms of molecular interaction between the components of the mixtures. Further, some more acoustical parameters such as relaxation time (τ), absorption coefficient (α/f ²) and relaxation strength (r) are calculated with various percentage of pectin has been studied which reveals interaction between the solvent and the polymer (pectin) at various concentration of the polymer. The observed values suggested the solute-solvent interaction is favored.     

Electrochemical impedance of passive iron in sulfuric acid water-organic solutions

The passive state of iron in sulfuric acid solutions in dimethylformamide and dimethyl sulfoxide containing different amounts of water is studied by the impedance method in a wide range of ac frequencies and also by the methods of polarization curves and open-circuit potential transients. In the studied systems, the metal passivation potential is shown to shift in the positive direction with a decrease in the water content in the solvent. The open-circuit potential of the destruction of the passive layer (Flade potential) is independent both of the nature of the solvents and the water amount in them. The impedance characteristics of passive iron can adequately be modeled by an equivalent circuit consisting of two parallelR-C circuits, i.e. similarly to a circuit used earlier for modeling the impedance of a passive Fe electrode in neutral aqueous solutions. The physical meaning of the equivalent-circuit elements calculated from the impedance measurements is analyzed. The effects the electrode potential, the organic-solvent type, the water content in the latter, etc. have on the elements are discussed. The obtained results suggest that the potential difference applied to the interface between passive iron and studied solutions is located mainly in the Helmholtz layer. Dedicated to the ninetieth anniversary of Ya.M. Kolotyrkin’s birth.     

A study of the electrical behaviour of isolated tomato cuticular membranes and cutin by impedance spectroscopy measurements

Different isolated tomato fruit cuticular membranes (ripe and green tomato cuticles and the cutin of these membranes) were studied by impedance spectroscopy measurements when the membranes were in contact with NaCl solutions at different concentrations. Remarkable differences in the impedance plots and the equivalent circuits associated to each membrane sample were obtained: the ripe tomato cuticle and the cutin, only present a relaxation process, but for the green tomato cuticle two relaxation processes were obtained. Using the equivalent circuits as models, electrical and electrochemical parameters for each membrane were determined. These results permit us to assign the relaxation processes to the different components of the tomato membrane (polyester matrix, carbohydrates and pigments), obtaining in this way a detailed picture of the different environments of the plant interface. Variation with NaCl concentration for the different electrical parameters was also studied, and the electrical resistance of the biopolymer matrix was obtained.     

Impact of sensitization on dissolution process of AISI 304 stainless steel during intergranular corrosion evaluated using DEIS technique

The paper presents the results of instantaneous impedance changes measurements vs. reactivation potential performed by means of dynamic electrochemical impedance spectroscopy (DEIS) technique for AISI 304 stainless steel (SS) dissolution process during intergranular corrosion (IG) in 0.5 M H₂SO₄ + 0.01 M KSCN solution. With the use of DEIS method, it was possible to estimate dynamic changes of the examined system’s impedance in conditions of proceeding IG process. Furthermore, the paper proposes an alternative way of evaluating AISI 304 stainless steel dissolution rate during intergranular corrosion based on approximation to theory of iron dissolution in sulfuric acid medium. Simultaneously, based on the DEIS measurements, information about the degree of sensitization of the examined material were obtained. Performed research revealed the advantages of the DEIS technique over the classical double-loop electrochemical potentiokinetic reactivation tests when investigating intergranular corrosion process.     

Acoustical studies on fructose with enzyme amylase in aqueous media at 298.15 K

Ultrasonic velocity (U), density (ρ), and viscosity (η) measurements have been carried out in three ternary mixtures of fructose with amylase in aqueous medium at 298.15 K. The experimental data have been used to calculate some derived parameters such as acoustical impedance (Z), relative association (R _a ), Rao’s constant (R _R), Wada’s constant (W), relaxation time (τ), relaxation amplitude (α/f ²), relaxation strength (r), and some excess thermodynamical properties like excess adiabatic compressibility (β ^E ), excess free length (L _f ^E ) excess free volume (V _f ^E ), excess internal pressure (π _i ^E ), and excess acoustical impedance (Z ^E ). The above parameters have been evaluated and discussed in light of molecular interactions in the mixture.     

Comparative kinetic study of decomposition of some diazepine derivatives under isothermal and non-isothermal conditions

Thermal analysis is one of the most widely used methods for studying the solid state of pharmaceutical substances. TG/DTG and DSC curves provide important information regarding the physical properties of the pharmaceutical compounds (stability, compatibility, polymorphism, kinetic analysis, phase transitions etc.). The purpose of a kinetic investigation is to calculate the kinetic parameters and the kinetic model for the studied process. The results are further used to predict the system’s behaviour in various circumstances. A kinetic study regarding the diazepam, nitrazepam and oxazepam thermal decomposition was performed, under non-isothermal and isothermal conditions and in a nitrogen atmosphere, for the temperature steps: 483, 498, 523, 538 and 553 K. The TG/DTG data were processed by three methods: isothermal model-fitting, Friedman’s isothermal-isoconversional and Nomen-Sempere non-parametric kinetics. In the model-fitting methods the kinetic triplets (f(α), A and E _a) that defines a single reaction step resulted in being at variance with the multi-step nature of diazepines decomposition. The model-free approach represented by isothermal and non-isothermal isoconversional methods, gave dependences of the activation energies on the extent of conversion. It is very difficult to obtain an accord with the similar data which resulted under non-isothermal conditions from a previous work. The careful treatment of the kinetic parameters obtained in different thermal conditions was confirmed to be necessary, as well as a different strategy of experimental data processing.     

A numerical Kramers-Kronig transform for the calculation of dielectric relaxation losses free from Ohmic conduction losses

 A numerical Kramers–Kronig transform is described which allows the calculation of dielectric relaxation losses from dielectric constant data measured at a limited set of frequencies differing by a factor of 2. Conversion formulas for both the central frequencies and for frequencies near the edges of the experimental frequency window are derived. The approach used can be extended easily to measurement frequencies with a different logarithmic spacing. Using this conversion, relaxation and dissipative, conduction losses can be separated. In this way Ohmic conduction processes and simultaneously occurring relaxation processes like dipole or space-charge relaxations can be analysed independently. The results of some simulations and of calculations on experimental data for poly(vinyl-chloride) are used to illustrate the potentials of the ɛ′ to ɛ″ conversion. Received: 17 May 1996 Accepted: 16 August 1996     

Macrokinetics of processes in glucose oxidase—peroxidase bienzyme systems when assaying glucose by the H2O2 reduction current: Effect of the enzyme ratio on the steady-state potential and H2O2 reduction current in glucose solutions of different concentrations

The effect of the enzyme ratio on the steady-state potential and H₂O₂ reduction current at a bienzyme glucose oxidase-peroxidase electrode in glucose solutions of different concentrations is studied. The derived equations adequately describe the experimental data and allow one to estimate characteristics of bienzyme electrodes in the linear domain of the current variation with the glucose concentration and the system’s sensitivity     

Kinetic studies of urban solid residues and leachate from sanitary landfill

Urban solid residues are constituted of food remaining, grass leaves, fruit peelings, paper, cardboard, rubber, plastic, etc. The organic fraction formed represents about 50% during the decomposition yields biogas and leachate, which are sources of pollution. Residue samples were collected from the landfill in different and cells from several ages and the corresponding leachate, both after treatments, were submitted to thermal analysis. Kinetic parameters were determined using Flynn–Wall–Ozawa method. The linear relation between the two kinetic parameters (ln A and E) was verified for organic residue urban’s samples, but not for leachate’s sample. The occurred difference can be attributed to the constituents present in leachate.